One problem met in local area networks (LANs) using time division multiple access (TDMA) is that when the number of users becomes large such as several hundreds on the same network the bit rate must be increased to provide sufficient bandwidths to all users. This results in that electronic circuits operating at fairly high speed must be used everywhere in the network, even if the actual bandwidth used by an individual node or terminal in the network is much smaller.
One way to alleviate this problem is to use wavelength division multiplexing (WDM) in a fiber-optical network but it can be costly since it usually means that tunable lasers, tunable filters and wavelength stabilization must be employed. Another solution which has been proposed is to use some form of spread-spectrum technique in the optical domain, see M. E. Marhic, “Coherent optical CDMA networks”, J. Lightwave Technology, Vol. 11, No. 5/6, May/June 1993, pp. 854-864, the published European patent application 0 027 413 having the title “Système de transmission multiaccès integral simultanè sur ligne de transmission par fibres optiques” for J. Foucard, which corresponds to U.S. Pat. No. 4,335,463, the published European patent application 0 240 124 having the title “Lightwave communication system” for G. J. Foschini and G. Vannucci, U.S. Pat. No. 5,519,526 for P. Chaa et al. having the title “Optical protocols for communication networks”. This is also called code division multiple access (CDMA). There are several problems related to the proposed solutions. They often require the use of short pulses and/or special devices for spectral encoding and decoding as in the solutions disclosed in the cited paper by Marhic and in the cited U.S. patent. These are not standard components and will thus be expensive. Another solution proposed in the cited European patent application 0 240 124 still requires a tunable optical filter to lock onto the emitting laser wavelength.
Most of the existing articles and patents, see for example the cited paper by Marhic, the cited European patent application 0 240 124 and U.S. Pat. No. 5,519,526, elaborating on the use of CDMA or spread-spectrum techniques for communication on optical fibers, use spreading of the optical spectrum. This means in general that the despreading also has to be done optically. It can be difficult and also expensive in terms of components. However, in the cited European patent application 0 027 413 a system is disclosed in which the spreading is done in the electrical domain, but in which the signal is kept in the baseband, without any subcarrier and/or control channel.
In the cited European patent application 0 240 124 the spreading is thus done in the optical domain, and although there is no wavelength control at the transmitter, a scanning tunable filter is required in the receiver to lock on an incoming signal and establish a connection.
In the cited U.S. Pat. No. 5,519,526 a spectral encoding of the optical signal is used which requires the use of very short pulses (to get enough spectral width) and a reconfigurable phase plate (for the spectral encoding). This will not be easy or cheap in practice.
The Japanese patent application JP 5/268658 discloses a system for transmission between an exchange station and plural base stations in a mobile telephone network using CDMA. The electrical RF signals from the exchange station are converted to optical form and transmitted on optical fibers to the antennas of the base stations.
In the published British patent application GB 2 125 253 a data communication system is disclosed using code division multiple access spread spectrum techniques. A message is modulated on to a quasi-random bit stream. The modulation technique used is also called pseudo-noise modulation and at the transmitter a modulated RF carrier is subjected to irregular phase reversals in a modulator. The reversals are controlled by a signal from a code generator producing a pseudo-random bit sequence, particular to each destination. The modulation data is superimposed on this bit sequence. The resulting waveform is applied to a balance modulator, which leaves the RF carrier unchanged for a 0 bit and produces a phase reversal for a 1 bit. The balance modulator is a liner driver, which can feed an optical fiber cable.